ipc: Change the timing of the construction of the attachment broker.

This CL ensures that the privileged attachment broker is constructed earlier in
the process of creating a child process. This fixes a race condition for single
process mode where the child thread's attachment broker would become the global
attachment broker.

BUG=534539

Review URL: https://codereview.chromium.org/1367013003

Cr-Commit-Position: refs/heads/master@{#350899}
2 files changed